This coin might be from the Cuzco Hoard, but I have no documentation. It is a scarcer date, the first date with the proper bust of Charles IV at Potosi. But something is going on with the reverse shield. There are two lions high stepping (rampant) in the lower left quadrant. Obviously this is a situation of double punching the lions during the creation of the die. Here's the coin. KM 73 26.8 grams Otherwise, there isn't anything very unusual about this coin. The grade is decent, but there are a couple of light scratches on the obverse. Krause does not mention this mint error for 1791. Perhaps this is a situation where it might be fairly common, if you know where to look. Has anyone else encountered this error?
Here's a Potosí mint 8 reales with a strongly doubled portrait. Lacking doubling in the legends, it's also likely that just the king punch was doubled on the die.
Beautiful coin! I guess it is not terribly surprising that doubling like this occurs in these early milled coins. Quality control, particularly in a remote mint such as Potosi, was probably lacking. What grade is your coin assigned? The strike and surfaces are excellent! I know that many years ago, in the mid 1980's as i recall, a large group of Charles III Potosi 8 reales hit the market, many in choice uncirculated condition. Perhaps this coin is one of them.
This one is graded MS61. I've only had it in my collection since 2013 and no provenance was provided in the Heritage auction listing.
Those are some lovely examples of type, @robinjojo and @jgenn I thought I had one of these 1791's, but this one was minted in Lima, Peru. Mine does not have any doubling, but it does have a lot of chopmarks. As an added bonus it has been heavily polished and put in a plastic box (Postal Commemorative Society or some such company): Thanks to its very unoriginal mirror-like surfaces, it was rather hard to photograph. It weighs 26.69 grams. Here it is in its plastic box: